Except for limestone, dolomite is the most abundant naturally occurring carbonate. Because of its composition, typically, 30.4 wt. % CaO, 21.7 wt. % MgO, and 47.9 wt. % CO.sub.2, dolomite is an important natural resource for preparing the compounds of magnesium and calcium as well as carbon dioxide. Among the calcium compounds, calcium carbonate is perhaps the most broadly used, finding abundant applications in the paper, pharmaceutical, cosmetic, food, chemical and cement industries, and as a pigment in plastics, rubber, dyes and paint. Among the compounds of magnesium, magnesium oxide is of extraordinary importance and is utilized as a fundamental component of basic refractories, as a material in the electrochemical industry, as a catalyst or catalyst carrier in the crude oil industry, and is utilized in paper and pulp industry in the so called magnesium bisulphite process of cellulose production, as well as many other applications including the cement, plastics and pharmaceutical industries.
Commercial methods are known and utilized for producing magnesium oxide, the hydrates thereof, as well as calcium carbonate from the above noted carbonate and oxide raw materials. These prior art processes are, however, rather complicated chemically and require an extensive investment to build a commercial plant for the production of these products.
It is well known, for example, that magnesium oxide and calcium carbonate can be produced from dolomite and magnesian limestone by a partial thermal decomposition thereof at a first stage temperature up to about 800.degree. C. depending upon the reaction conditions. These two reaction products, however, are produced in a powdery admixture from which they cannot be separated by any known physical process. While there are several known chemical processes for separation of the two constituents, most are based on the selective dissolution of the magnesium oxide, whereby only calcium carbonate is recovered directly, leaving the dissolved magnesium oxide to be further processed to produce various compounds thereof. For example, one known process utilizes solutions of ammonium carboxylates and particularly ammonium formate, to dissolve the magnesium oxide, and within which calcium carbonate is insoluble. The magnesium containing solution is thereafter further processed to produce magnesium hydroxide, (see Czechoslovakian Inventors Certificate No. 180,284) magnesium basic carbonate, (See Czechoslovakian Inventors Certificate No. 196,905); magnesium carbonate trihydrate (See Czechoslovakian Inventors Certificate No. 194,644); and the like, from which magnesium oxide can thereafter be produced if desired. In accordance with Czechoslovakian Inventors Certificate No. 199,323, there arises during the extraction by aqueous solutions of ammonium sulphate, nitrate or chloride, the corresponding magnesium salts which are subsequently converted to magnesium basic carbonate or oxide. These compounds exist among the products of magnesium oxide manufacturers. Other methods of extracting magnesium dolomite and dolomitic limestone have been disclosed in Rumanian Patent Specification Nos. 55,712 and 59,779.
The thermal decomposition of dolomite at a second stage temperature exceeding 900.degree. C., produces a mixture of oxides, theoretically about 41.7 wt. % MgO and 58.3 wt. % CaO. As before, these two oxides cannot be separated by any known physical process. A chemical process is known for increasing the magnesium oxide content whereby the admixture is reacted with nitric acid, ammonium nitrate, ammonium formate or ammonium acetate, to remove merely a portion of the calcium oxide from the insoluble residue (See Czechoslovakian Inventors Certificate No. 190,976);. As a result, the residue will have an increased magnesium oxide content consisting of about 26.8 to 43.0 wt. % CaO and 52.6 to 72.4 wt. % MgO The extracting solution can be further processed with ammonia and carbon dioxide to produce calcium carbonate and the corresponding ammonium salt.
These and the other prior art processes all tend to suffer certain disadvantages, such as effecting only a partial separation, or being contaminated with by-product impurities thus necessitating further refining efforts. In addition to multiple processing steps, even the simple, basic processing steps are complicated rendering difficulties in the processes, such as difficulty in processing, including such difficulties as poor filterability, washability and dehydration and the like.